Subjects must not have a relapse during 30 days before initiation of treatment. If a relapse occurs during the last 30 days of the pre-treatment baseline period and eligibility MRI criteria are fulfilled, beginning of the treatment (day 1) is delayed for at least so many days that treatment starts not earlier than 30 days after the relapse and not earlier than 60 days in case i.v. corticosteroids had been given. Similarly, the baseline needs to be prolonged if corticosteroid treatments become necessary during these three months.